1. Field of the Invention
This invention relates to an improvement in the recovery of precious metals such as gold and silver and in particular to the recovery of gold from aqueous cyanide solutions thereof. The recovery is achieved by contact of the aqueous cyanide solution containing the precious metals, particularly gold, with an ion exchange resin containing a guanidine functionality. The guanidine reagent extracts the gold from the aqueous solution and the gold is then subsequently eluted or stripped from the guanidine reagent and recovered by conventional methods. The invention also relates to certain novel guanidine compounds which are suitable for extracting gold from cyanide solutions.
2. Description of Related Art
Gold occurs primarily as the native metal, alloyed with silver or other metals or as tellurides. It is commonly associated with the sulfides of iron, silver, arsenic, antimony and copper. Silver occurs as finely disseminated metal in rocks of hydrothermal origin as silver chloride, sulfide or tellurides and as complex sulfides with antimony and arsenic. Historical practice with ores containing native metal involves crushing, concentration of the gold or silver by gravity separation and recovery by amalgamation with mercury. Environmental concerns have resulted in abandonment of this process in most cases. Currently there are two major processes for recovery of gold and/or silver. The most widely accepted processes today involve leaching with caustic cyanide solution coupled with recovery of the metal values by concentration with zinc dust (Merrill-Crowe) or concentration of the gold and silver cyanide complexes by absorption on charcoal (carbon absorption scheme) also referred to as Carbon in Column (CIC) or Carbon in Pulp (CIP). A carbon process is also described in U.S. Pat. No. 5,073,354 in which gold and silver also on activated carbon is stripped employing a caustic-benzoate strippent. Another process recently practiced in the Soviet Union is one in which quaternary amine ion exchange resins are employed as a replacement for charcoal in the carbon absorption scheme.
In a recent publication "Selectivity Considerations in the Amine Extraction of Gold from Alkaline Cyanide Solutions" by M. A. Mooiman and J. D. Miller in "Minerals and Metallurgical Processing", August 1984, Pages 153-157, there is described the use of primary, secondary and tertiary amines to which have been added certain Lewis base modifiers such as phosphorus oxides and phosphate esters for the extraction of gold from alkaline cyanide solutions.
Clarified leach liquors containing the gold are obtained by leaching with cyanide solutions through either the dump or heap leaching techniques. In heap leaching, the ore is placed on specially prepared impervious pads and a leaching solution is then applied to the top of the heap and allowed to percolate down through the heap. The solution containing the dissolved metal values eventually collects along the impervious pad and flows along it to a collection basin. From the collection basin, the solution is pumped to the recovery plant. Dump leaching is similar to heap leaching, in which old mine waste dumps which have sufficient metal value to justify processing are leached in place. The gold in clarified leach solutions may be recovered by direct precipitation in the Merrill-Crowe process, or by adsorption on Charcoal in Columns (CIC), followed by either electrowinning or by precipitation in the Merrill-Crowe process.
In certain conditions, unclarified solutions are generated by agitated vat leaching. In this continuous Carbon in Pulp (CIP) leaching process, the ore is slurried with agitated leach solution in the presence of carbon granules to generate a pulp. Dissolved gold is adsorbed onto the carbon resulting in low aqueous gold concentrations, which often increases the rate and completeness of gold extraction from the ore. Carbon granules carrying the gold are separated from the pulp by screening, and the gold is recovered from the carbon typically by elution with hot sodium hydroxide solution followed by electrowinning. Before the carbon granules can be returned to the leaching step, they must be activated by hazardous and expensive washing and heating steps. Coconut shell activated carbon is preferred, but is expensive.
Different amine functionalities have been considered in the past in both the liquid/liquid extraction and liquid/solid extraction of gold. In the case of liquid/solid extraction, aurocyanide is too strongly bound by the quaternary amine functionality of the resins, so that stripping is difficult and requires special treatment. In addition, no selectivity of metal cyanide complexes and leach liquors is shown. Resins with weaker basic amine functionalities cannot perform well in the pH range (10-11), the pH of the common leach liquors. For liquid/liquid extraction such as the work of Mooiman and Miller, organophosphorus modifiers, i.e. trialkylphosphates, are required to increase the amine basicity in order to permit efficient extraction of the gold materials. These materials must be used in large amounts. These systems still do not extract adequately at the typical pH of leach liquors.
In commonly assigned U.S. Pat. Nos. 4,814,007, 4,992,200 and 4,895,597 there is described the use of guanidine compounds for extracting precious metals particularly gold from aqueous alkaline cyanide solutions. Specific guanidine compounds disclosed therein are certain di-alkyl guanidines such as di-n-octyl, di-2-ethylhexyl and di-tridecyl guanidines employed in a liquid/liquid solvent system. In a solid/liquid system, an ion exchange resin carrying guanidyl functionality was employed, specifically a butyl hexyl guanidine carried on a chloromethylated polystyrene resin having a divinylbenzene content, for example, of 2%. In general the guanidine compounds had the formula ##STR1## where R.sub.1 through R.sub.5 are H, an ion exchange resin carrier or a hydrocarbon group having up to 25 carbon atoms. Generally, the guanidine compounds are to have a pKa greater than 12, and preferably should be greater than 13. The patent cautions against having more than one aromatic group, such as phenyl, as such groups tend to decrease the basicity to a level below a pKa of 12. Thus, the groups should be selected to provide guanidine compounds having a pKa preferably above 13. In solutions containing gold, silver and copper, selectivity experiments showed a general preference of gold over silver or copper.
In commonly assigned U.S. Pat. No. 5,028,259, an improvement is described in which an ion exchange resin, carrying certain guanidyl functionality from a methyl substituted guanidine, extracts precious metals and provides for increased selectivity, particularly for gold. In the generalized formula above, when one of the R groups, R.sub.1 through R.sub.5, is an ion exchange resin carrier, at least one of the remaining R groups is an aliphatic hydrocarbon group having 1-25 carbon atoms and when other than methyl at least 3 of the R groups are hydrocarbon. Specific resins described are N-methyl guanidine resin, N,N-dimethyl guanidine resin and tetramethyl guanidine resin.
In South African Patent 71/4981 the use of guanidines on a resin for extraction of gold from aqueous acidic solutions is described. While general reference is made to alkyl substituted guanidines in which the alkyl group contains 1-6 carbon atoms, the specific resin employed used an unsubstituted guanidine.
In South African Patent 89/2733, a similar process is described using resins containing guanidyl functionality for recovering gold from aqueous alkaline cyanide solutions. In the generalized guanidine formula the R groups are described as H, alkyl or aryl. In the specific examples, the chloromethylated resin carrier is aminated with guanidinium hydrochloride or nitrate providing a guanidine resin, in which no R groups are hydrocarbon.
The references discussed above accordingly teach that where the guanidine is other than guanidine itself, the hydrocarbon substituents should be such as to provide a pKa of preferably greater than 13, and at least greater than 12. Since aromatic groups such as phenyl, may lower the pKa to less than 12, the substituents must be selected to provide the pKa greater than 12, and preferably greater than 13.